The present invention relates to a mass spectrometer, especially to the ion optical system for transporting ions generated in an ion source to a mass analyzer such as a quadrupole mass filter.
Among various mass spectrometers, the Electrospray Ionization Mass Spectrometer (ESI-MS), the Atmospheric Pressure Chemical Ionizing Mass Spectrometer (ACPI-MS) and Radio-frequency Induction Plasma Mass Spectrometer (ICP-MS) are called atmospheric pressure type mass spectrometers (API-MS) because the sample is ionized under almost atmospheric pressure.
FIG. 9 is a schematic sectional view of a conventional ESI-MS, which includes the ionizing chamber 1 and the analyzing chamber 9. In the ionizing chamber 1, a nozzle 2 is provided which is connected to the exit of, for example, a liquid chromatographic column. In the analyzing chamber 9, a quadrupole filter 10 and an ion detector 11 are provided. Between the ionizing chamber 1 and the analyzing chamber 9, the first vacuum chamber 4 and the second vacuum chamber 7 are placed, where air-tight walls separate those chambers 1, 4, 7, 9. The ionizing chamber 1 and the first vacuum chamber 4 communicate with each other only with a desolvation tube 3 provided in the wall between them, where the desolvation tube 3 has a narrow conduit at its center. The first vacuum chamber 4 and the second vacuum chamber 7 communicate with each other only with a skimmer 6 provided in the wall between them, where the skimmer 6 has a very narrow orifice.
The pressure in the ionizing chamber 1, which is the ion source, is almost atmospheric due to the vaporized molecules of the liquid sample continuously supplied from the nozzle 2. The pressure of the first vacuum chamber 4 is lowered by a rotary pump to about 102 Pa, that of the second vacuum chamber 7 is lowered by a turbo molecular pump to about 10xe2x88x921 to 10xe2x88x922 Pa, and that of the analyzing chamber 9 is made as low as 10xe2x88x923 to 10xe2x88x924 Pa by a turbo molecular pump. Thus the pressures of those chambers are gradually decreased from the almost atmospheric pressure of the ionizing chamber 1 to the very high vacuum of the analyzing chamber 9. This multi-stage differentiated evacuation system assures the high vacuum of the analyzing chamber 9.
The liquid sample is sprayed from the tip of the nozzle 2 into the ionizing chamber 1, wherein the sample is electrically charged (electrosprayed). When the solvent in the sprayed droplets evaporates, the sample molecules are ionized. The droplets containing such ions are drawn into the desolvation tube 3 due to the pressure difference between the ionizing chamber 1 and the first vacuum chamber 4. Since the desolvation tube 3 is heated, the solvent in the droplets further evaporates and the sample molecules are further ionized. A first ion lens 5, which may be constructed by a cylindrical electrode, is provided in the first vacuum chamber 4. The first ion lens 5, with the electric field created in it, assists the drawing-in of the ions coming through the desolvation tube 3, and converges the ions to the orifice of the skimmer 6.
The ions introduced into the second vacuum chamber 7 through the orifice of the skimmer 6 are converged and accelerated by the second ion lens 8, which may be constructed by concentrically arrayed ring electrodes, and sent to the analyzing chamber 9. In the analyzing chamber 9, only such ions that have a certain mass to charge ratio can pass through the central space of the quadrupole mass filter 10, and other ions dissipate while traveling through the space. The ions that have passed through the quadrupole mass filter 10 enter the ion detector 11, which outputs an electrical signal corresponding to the number of ions detected.
In the above construction, the first ion lens 5 and the second ion lens 8 are generally called ion optical systems, whose primary functions are to converge flying ions with their electric fields, and, in some cases, accelerate them toward the next stage. Conventionally, various types of ion optical systems have been used or proposed.
FIG. 10 shows a multi-rod type ion lens 20, which has four rods. The number of rods can be six or eight, for example, and generally it can be any even number no less than four. To any neighboring two rods among the rods of an even number, the same DC voltage plus the same RF (radio-frequency) voltages having opposite polarities are applied. Ions introduced along the central axis (xe2x80x9cion optical axisxe2x80x9d) C of the space surrounded by the rods travel through the space vibrating at the frequency the same as that of the RF voltage. This structure has a better ion converging efficiency, so that larger number of ions can be passed onto the next stage.
In the multi-rod type ion lens 20, however, the inscribing circle P1 (which contacts the inner surfaces) of the rods 201-204 at the entrance and the inscribing circle P2 at the exit have the same diameter, and thus the ion traveling space surrounded by the rods 201-204 is shaped almost cylindrical. As shown in FIG. 9, especially in the first vacuum chamber 4, ions ejected from the desolvation tube 3 spread conically, so that the capturing efficiency of the first ion lens 5 having a rather small entrance is rather low. If the entrance of the multi-rod type ion lens 20 is broadened, however, the converging efficiency of ions toward the orifice becomes low, on the other hand, so that the overall ion passing efficiency cannot be improved. Since, further, the value of the DC voltage is constant on the ion optical axis C, ions are not accelerated in the space. Thus, in the first vacuum chamber 4 where the pressure is rather high, compared to the low pressure or high vacuum in the following chambers, ions are deprived of their kinetic energy due to collisions with the remaining gas molecules, and fewer ions can pass through the firs ion lens 5.
Addressing the problem, the present applicant proposed a new ion lens in the Publication No. 2000-149865 of unexamined Japanese patent application. FIG. 11 shows an example of the new ion lens 21, where virtual rod electrodes 211-214 are used. A virtual rod electrode is composed of a plurality of metal plate electrodes aligned in a row along the ion optical axis C, where every metal plate is positioned substantially vertical to the ion optical axis C. Owing to such a construction of the virtual rod electrodes 211-214, the plate electrodes can be arranged as shown in FIG. 11, where they are arranged closer to the ion optical axis C toward the exit of the virtual rod electrodes. Since, in this case, the ion passing space is conical with a broader entrance, more ions can be collected at the entrance and are gradually converged to the narrower exit by the electric field produced by the virtual rod electrodes. Thus the transporting or passing efficiency of ions is improved.
Further, since different voltages can be applied to the respectively independent plate electrodes constituting a virtual rod, a static electric field having a gradient can be produced, and the ions can be accelerated.
Though the virtual rod electrodes as described above have such advantages, it is necessary to set and arrange respective plate electrodes to the proper positions, and the holding or fixing structure is rather complicated and rather cost-inefficient.
The present invention addresses the problem. An object of the present invention is therefore to provide an ion optical system having a simpler structure and high ion passing efficiency.
According to the present invention, an ion optical system for converging ions includes:
an ion lens composed of platelet electrodes of an even number no less than four arranged radially and symmetrically around an ion optical axis connecting the ion source and the mass analyzer; and
a voltage generator for applying a voltage composed of a DC voltage and an RF voltage to a group of alternately arranged platelet electrodes and for applying another voltage composed of the same DC voltage and another RF voltage having the same frequency and an opposite polarity to the other group of alternately arranged platelet electrodes.
Therefore, a mass spectrometer according to the present invention includes:
an ion source;
a mass analyzer for analyzing ions generated by the ion source with their mass to charge ratio;
an ion lens composed of platelet electrodes of an even number no less than four arranged radially and symmetrically around an ion optical axis connecting the ion source and the mass analyzer; and
a voltage generator for applying a voltage composed of a DC voltage and an RF voltage to a group of alternately arranged platelet electrodes and for applying another voltage composed of the same DC voltage and another RF voltage having the same frequency and an opposite polarity to the other group of alternately arranged platelet electrodes.
In the mass spectrometer of the present invention, when ions are introduced into the ion traveling space defined by the inner surfaces of the platelet electrodes, the ions travel along the ion optical axis and converge to a rear focal point of the ion lens, while they are vibrated by the above-described voltages applied to the platelet electrodes. By placing a small hole or orifice communicating to the next chamber at the rear focal point of the ion lens, larger number of ions can be sent to the next chamber, which improves the sensitivity and precision of the mass spectrometer.
A platelet electrode of the ion lens of the present invention corresponds to a rod of the conventional multi-rod type ion lens. In the present invention, the outer edge of the platelet electrode can be any shape convenient for fixing. For example, the outer edge can be a flat face, which is convenient for screw fixing. This simplifies the structure of the ion lens, and decreases the cost while maintaining the high ion passing efficiency.
A preferable variation of the ion lens of the present invention is to cut off a front corner of every platelet electrode. This makes the inscribing circle of the platelet electrodes at the entrance of the ion lens larger than that at the exit, which means that ions enter into a large entrance, and converge as they travel along the ion optical axis to the small exit. This enhances the ion passing efficiency onto a small hole or orifice communicating to the next chamber.
The cutting line of the corner cut-off is not limited to a straight line, but it can be curved as long as the inscribing circle becomes monotonously smaller as the ions progress.
Another variation of the ion lens of the present invention is to use an electrically insulating material for the platelet electrodes, and to form an electrically resistive layer on the inner surface of every platelet electrode. Then a pair of conductive layers are formed on the front edge and on the rear edge of every platelet electrode, wherein a pair of voltages composed of the same RF voltage and different DC voltages are applied to the front edge conductive layer and the rear edge conductive layer respectively.
Still another variation of the ion lens of the present invention is to use a semiconducting material for the platelet electrodes. In this case, no electrically resistive layer is necessary on the inner surface of every platelet electrode. A pair of conductive layers are also formed on the front edge and on the rear edge of every platelet electrode, wherein a pair of voltages composed of the same RF voltage and different DC voltages are applied to the front edge conductive layer and the rear edge conductive layer respectively.
In those ion lenses, due to the difference in the DC voltages applied to the front and rear edges, a voltage gradient is produced in the inner surface of every platelet electrode along the ion optical axis. The voltage gradient of the platelet electrodes surrounding the ion traveling space produces a potential gradient in it, which gives ions kinetic energy and accelerates them. This decreases the possibility of dissipation of ions due to loss of kinetic energy, and enhances the ion passing efficiency.
The ion lens of the present invention is suitable especially for such a type of mass spectrometer that ions spread broadly in the entrance or ions tend to lose kinetic energy due to collisions with remaining gas molecules in a rather low vacuum. Thus the ion lens of the present invention is suited to be used in a mass spectrometer in which:
the ion source is placed in a chamber of almost atmospheric pressure;
the mass analyzer is placed in a chamber with a high vacuum;
a plurality of intermediate vacuum chambers are placed between the ion source chamber and the mass analyzer chamber; and
the ion lens is placed in a chamber adjacent to the ion source chamber.